Age at menarche, time to regular cycling, and breast cancer (North Carolina, United States)

Objectives: Later menarche, and a longer time until onset of regular cycling, are considered markers of lower lifetime exposure to circulating ovarian hormones, While later age at menarche is associated with reduced breast cancer risk, evidence for the relationship between time until onset of regular cycles and breast cancer is inconsistent. We evaluated both associations with breast cancer risk. Methods: We used data from the Carolina Breast Cancer Study, a population-based case-control study of breast cancer among White and Black women aged 20 to 74 years, residing in central and eastern North Carolina (United States). Cases were diagnosed between May 1993 and June 1996, Unconditional logistic regression models were limited to women with complete data for the risk factors considered (n = 830 cases, 758 controls). Results: We observed an inverse relationship between age at menarche and breast cancer risk, but found little support for the hypothesis that a longer time until onset of regular menstrual cycling was associated with reduced risk of breast cancer (odds ratios = 1.0 [95 percent confidence interval (CI) = 0.7-1.5], and 1.2 [CI = 0.8-1.6], respectively, for 1-4 years and < 1 year until onset of regular cycling, relative to 5+ years). There was little relationship between age at menarche and time until regular cycling. We found strong evidence that delays in onset of regular cycling were associated with increased frequency of irregular cycles throughout young adulthood. Conclusions: Given the inconsistent findings regarding the links between menstrual cycle characteristics and breast cancer, and recent recommendations to delay menarche and alter the patterns of cycles of young women in order to reduce breast cancer risk, this topic calls for further, innovative study.